Kaposi's Sarcoma is a disfiguring and potentially fatal form of hemorrhagic sarcoma. It is characterized by multiple vascular tumors that appear on the skin as darkly colored plaques or nodules. At the histological level, it is characterized by proliferation of relatively uniform spindle-shaped cells, forming fascicles and vascular slits. There is often evidence of plasma cells, T cells and monocytes in the inflammatory infiltrate. Death may ultimately ensue due to bleeding from gastrointestinal lesions or from an associated lymphoma. (See generally Martin et al., Finesmith et al.)
Once a relatively obscure disease, it has leapt to public attention due to its association with AIDS. As many as 20% of certain AIDS-affected populations acquire Kaposi's during the course of the disease. Kaposi's Sarcoma occurs in other conditions associated with immunodeficiency, including kidney dialysis and therapeutic inimunosuppression. However, the epidemiology of the disease has suggested that immunodeficiency is not the only causative factor. In particular, the high degree of association of Kaposi's with certain sexual practices suggests the involvement of an etiologic agent which is not the human immunodeficiency virus (Berel et al.).
A herpes-virus-like DNA sequence has been identified in tissue samples from Kaposi's lesions obtained from AIDS patients (Chang et al., confirmed by Ambroziuk et al.). The sequence was obtained by representational difference analysis (Lisitsyn et al.), in which DNA from affected and unaffected tissue were amplified using unrelated priming oligonucleotides, and then hybridized together to highlight differences between the cells. The sequence was partly identical to known sequences of the Epstein Barr Virus and herpesvirus saimiri. It coded for capsid and tegument proteins, two structural components. In a survey of tissues from various sources, the sequence was found in 95% of Kaposi's sarcoma lesions, regardless of the patients' HIV status (Moore et al.). 21% of uninvolved tissue from the same patients was positive, while 5% of samples from a control population was positive. There was approximately 0.5% sequence variation between samples. The sequence was also detected at a higher copy number in body cavity lymphoma, a lymphomatous effusion with a B-cell genotype occurring uniquely in AIDS patients (Cesarman et al.). Other AIDS-associated lymphomas were negative.
The herpes virus family comprises a number of multi-enveloped viruses about 100 nm in size, and capable of infecting vertebrates. (For general reviews, see, e.g., Emery et al., Fields et al.). The double-stranded DNA genome is unusually large--from about 88 to about 229 kilobases in length. It may produce over 50 different transcripts at various stages in the life cycle of the virus. In one of the stages, a number of nucleotide and polynucleotide processing enzymes are produced that are required for viral replication, including DNA polymerase, DNAse, dUTPase, ribonucleotide reductase, uracil-DNA glycosylase, and thymidine kinase. These functional proteins tend to be relatively well conserved between species, compared with external viral components (Karlin et al.).
The herpes virus family has been divided into several subfamilies. Assignments to each of the categories were originally based on the basis of biologic properties, and are being refined as genomic sequence data emerges. The alpha subfamily comprises viruses that have a broad host range, a short replicative cycle, and an affinity for the sensory ganglia. They include the human simplex virus and the Varicella-zoster virus. The beta subfamily comprises viruses that have a restricted host range, and include Cytomegalovirus and human Herpes Virus 6. The gamma subfamily comprises viruses that are generally lymphotrophic. The DNA is marked by a segment of about 110 kilobases with a low GC content, flanked by multiple tandem repeats of high GC content. The subfamily includes Epstein Barr Virus (EBV), herpes virus saimiri, equine Herpes Virus 2 and 5, and bovine Herpes Virus 4.
Herpes viruses are associated with conditions that have a complex clinical course. A feature of many herpes viruses is the ability to go into a latent state within the host for an extended period of time. Viruses of the alpha subfamily maintain latent forms in the sensory and autonomic ganglia, whereas those of the gamma subfamily maintain latent forms, for example, in cells of the lymphocyte lineage. Latency is associated with the transcription of certain viral genes, and may persist for decades until conditions are optimal for the virus to resume active replication. Such conditions may include an immunodeficiency. In addition, some herpes viruses of the gamma subfamily have the ability to genetically transform the cells they infect. For example, EBV is associated with B cell lymphomas, oral hairy leukoplakia, lymphoid interstitial pneumonitis, and nasopharyngeal carcinoma.
A number of other conditions occur in humans and other vertebrates that involve fibroproliferation and the generation of pre-neoplastic cells. Examples occurring in humans are retroperitoneal fibrosis, nodular fibromatosis, pseudosarcomatous fibromatosis, and sclerosing mesenteritis. Another condition known as Enzootic Retroperitoneal Fibromatosis (RF) has been observed in a colony of macaque monkeys at the University of Washington Regional Primate Research Center (Giddens et al.). Late stages of the disease are characterized by proliferating fibrous tissue around the mesentery and the dorsal part of the peritoneal cavity, with extension into the inguinal canal, through the diaphragm, and into the abdominal wall. Once clinically apparent, the disease is invariably fatal within 1-2 months. The condition has been associated with simian immunodeficiency (SAIDS) due to a type D simian retrovirus, SRV-2 (Tsai et al.). However, other colonies do not show the same frequency of RF amongst monkeys affected with SAIDS, and the frequency of RF at Washington has been declining in recent years.
The study of such conditions in non-human primates is important not only as a model for human conditions, but also because one primate species may act as a reservoir of viruses that affect another species. For example, the herpes virus saimiri appears to cause no disease in its natural host, the squirrel monkey (Saimiri sciureus), but it causes polyclonal T-cell lymphomas and acute leukemias in other primates, particularly owl monkeys.
There is a need to develop reagents and methods for use in the detection and treatment of herpes virus infections.
For example, there is a need to develop reagents and methods which can be used in the diagnosis and assessment of Kaposi's sarcoma, and similar conditions. Being able to detect the etiologic agent in a new patient may assist in differential diagnosis; being able to assess the level of the agent in an ongoing condition may assist in clinical management. The tegument encoding polynucleotide of Chang et al. may have limited applicability in this regard. It is desirable to obtain a marker capable of distinguishing active from latent infection. It is also desirable to obtain a marker that is immunogenic, and can be used to assess immunological exposure to the agent as manifest in the antibody response.
Second, there is a need to develop reagents and methods which can be used in the development of new pharmaceuticals for Kaposi's sarcoma, and similar conditions. The current treatment for Kaposi's is radiation in combination with traditional chemotherapy, such as vincristine (Northfelt, Mitsuyasu). While lesions respond to these modalities, the response is temporary, and the downward clinical course generally resumes. Even experimental therapies, such as treatment with cytokines, are directed at the symptoms of the disease rather than the cause. Drug screening and rational drug design based upon the etiologic agent can be directed towards the long-felt need for a clinical regimen with long-term efficacy.
Third, there is a need to develop reagents and methods which can be used to identify viral agents that may be associated with other fibroproliferative conditions. The representational difference analysis technique used by Chang et al. is arduously complex, and probably not appropriate as a general screening test. More desirable are a set of primers or probes to be used as reagents in more routine assays for surveying a variety of tissue samples suspected of containing a related etiologic agent. Preferably, the reagents are sufficiently cross-reactive to identify previously undescribed viral compounds, but sufficiently specific to avoid identifying unwanted viruses or endogenous components of the host.